Nark

So I went and laid down on the grass. While laying there I piped a reeler whom I knew. He had a nark (a policeman's spy) with him. So I went and looked about for my two pals, and told them to look out for F. and his nark.

the bystander. […] She thought you was a copper's nark, sir. / the note taker [with quick interest] Whats a copper's nark? / the bystander [inapt at definition] It's a—well, it's a copper's nark, as you might say. What else would you call it? A sort of informer.

When we got to the Chief of Police's office a crowd of the most dreadful-looking scoundrels, obviously police narks, informers, and spies of every kind, were hanging about outside the door.

“If you nark on me, I’ll rip your arms off,” said Tim to his brother, as he passed him a joint.

I don’t trust that guy; he’s gonna nark us out.

It really narks me when people smoke in restaurants.

He narks in my ear all day, moaning about his problems.

Nark it! I hear someone coming!